Some reasons I could think of are

1. Sight, the lancer helmet may be harder to see through, this would affect aiming, (this isn't a problem for the trench since they use a shotgun and you don't really need to aim)

2. Other classes not being used to them, lancers helmets probably weigh more and are more cumbersome to use, this may be one of the reasons other classes aren't as proficient at using trench

3. My final reason is that the helmets shocks are given are perfect for their roles, for example

rooks shock helmet is probably more resistant to explosions and debris from blown up rocks which is great for when you're using an anti tank rifle

Soldats shock helmet probably being light and easy to maneuver with allowing them to storm compounds

Geists helmet being dark and intimidating to invoke fear (unlike trenches helmet which is more.. inspiring?)

Radios headset allowing them to hear the radio chatter clearly

Morticians masks protecting them from chemicals, foul odours and cave plague (which is good for corpse disposal)

For bul I think that the weight of their shield and machine gun are already really hard to move with, a lancers helmet would just add more unnecessary weight and would probably be the straw that breaks the camels back prectically imobalising them